Make it more clear how to use infinite mode in game or in curseforge mod description please. There is no indication anywhere that you physically need one of the item and slot it into the replicator to make it work.

https://www.curseforge.com/minecraft/mc-mods/replication
Simple as change this sentence:
Replicators can be used in "Infinite Mode", where they will keep replicating a resource until it is full or has run out of matter, you can configure that mode in the GUI.
to
Replicators can be used in "Infinite Mode", where they will keep replicating a resource until it is full or has run out of matter, you can configure that mode in the GUI by placing the resource into the slot.

Done